In pressurized-water nuclear reactors, the core composed of fuel assemblies is surrounded by a partitioning consisting of plates which are arranged vertically and are contiguous so as to reproduce the shape of the external surface of the core. These vertical plates are held in position by horizontal reinforcing plates which occupy the entire width of the space between the core shroud and the partitioning. The adjoining edges of the vertical plates are not necessarily contiguous over their entire length, and leakage gaps may exist along these edges. As there is a difference in pressure between the internal space of the partitioning and the space between the core shroud and the external surface of the partitioning, jets of water subjected to a certain pressure can be directed from the outside of the partitioning towards the inside of the partitioning. These water jets thus strike the rods of the peripheral core assemblies. It is therefore desirable to limit as far as possible this phenomenon, which may give rise to premature wear of the fuel rods arranged at the periphery of the core.
The suggestion was made, in French patent application No. 81/14,899 filed on July 30, 1981 by the present applicant, to eliminate the leakage gaps along the edges of the vertical plates of the partitioning, by machining tapped holes in these plates at the point where they overlap and by introducing into these tapped holes screws which are then tightened sufficiently to bring together the edges of the plates until they are contiguous.
The suggestion was also made in this patent application for a device for machining the tapped holes and positioning and tightening the screws.
Such a device, which must perform different functions, consists of a barrel-type machine tool which is entirely contained in a leak-proof housing the front face of which has an opening through which the tools pass. This front face of the machine tool can be applied in a leak-proof manner against the partitioning walls on which machining is performed and the screw positioned. Such a machine tool may comprise a barrel with six positions, by means of which it is possible to bring six different tools, in succession, into the working zone, these tools being displaced axially by a jack and thus passing through the opening of the housing in a centered position. 0f the six tools, four which operate by rotating about their axes are driven simultaneously by the same hydraulic motor. However, only the tool in the working position is subjected to the thrusting action of the hydraulic jack in order to obtain its feed movement.
Using such a device, it is possible to work under water and without introducing machining chips into the reactor vessel.
The hydraulic rotary-drive motor and the hydraulic feed jack are supplied with fluid, the flow rate of which is controlled by means of elements which are electrically controlled by potentiometers. It is thus possible to regulate the supply rate of the jack and the hydraulic motor, thereby making it possible to adjust the feed speed of the jack and the rotational speed of the hydraulic motor. Each position of the potentiometers thus corresponds to an electrical current for controlling the elements of the hydraulic circuit supplying the jack and the hydraulic motor.
The supply pressure of the jack increases with the resisting load on the tool as far as a limit threshold. The torque exerted on the hydraulic motor increases as a function of the resisting torque.
However, if calibration of the machine tool has not been performed, the torques or axial loads corresponding to the various positions of the potentiometers are not known.
In particular, when the screwing tool is used, it is extremely important to determine to a high degree of accuracy the torque exerted on the screw, in order to know whether the residual tension in the screw is sufficient to keep the two partitions joined. It is also necessary to avoid exerting too great a torque on the screw, since this can cause it to break.
Until now, no device was known whereby it was possible to measure the dynamic torques exerted by a hydraulic motor on a set of tools which are used, for example, to tighten screws, and which exert variable resisting torques in accordance with their degree of clamping.
Similarly, no means were known for determining easily the torque and axial load exerted on a drilling, countersinking or tapping tool when used during machining. In the case where screw holes are machined in the partitions of a nuclear reactor core, these torques and loads may vary considerably, depending on the metallurgical condition of the material and the state of wear of the tools. It is thus very important to know these torques and loads so as to avoid or prevent breakage of the tools.
The screws used to eliminate the leakage gaps in a partitioning may be of the type described in the present applicant's French patent application No. 83/17,279. These screws have a fixing means for making them captive, consisting of balls which can be pushed towards the outside of the screw by a maneuvering rod so as to crush the threading of the tapped hole in which the screw is inserted. In this case, it is necessary to be able to control the thrusting load exerted on the maneuvering rod and hence the axial load of a tool for pushing the rod. In all cases, the screws arranged in the partitioning must be made captive by an element actuated by a maneuvering rod. The multi-purpose machine tool which performs machining and screwing must therefore also perform pushing and blocking of the screw maneuvering rod by means of a controlled axial load.